1. Field of the Invention
This invention relates generally to the fail safe switching of electrical power to an electric load, such as an electric motor driving a hydraulic pump of a wheel chair lift, through a pair of series connected power switches or contactors and more particularly relates to both the control of the sequential switching of such switches as well as the monitoring of the switch operating conditions in association with the sequential switching process in order to detect switch malfunctions.
2. Description of the Related Art
There are many types of machines that transport people or move mechanical apparatus in the vicinity of people or otherwise require reliable control so they do not malfunction and cause personal injury or property damage. One of the most common electrical loads associated with such machines is an electric motor that is or drives a prime mover to move the mechanical apparatus. Such machines should not only operate when they are signaled or otherwise commanded to operate, but of more critical importance to safety is that they stop operating when they are signaled or otherwise commanded to stop. Although the invention is applicable to a broad variety of machines with electrical loads that have such control and safety requirements, it is illustrated in connection with one such machine, a wheelchair lift having an electric motor driven hydraulic pump as its prime mover.
Many buses and vans are equipped with hydraulic wheelchair lift systems. In wheelchair lift systems, safety is probably the single most important factor. These lifts transport people who have a physical disability and it is particularly desirable to avoid jeopardizing them with apparatus that has the possibility of failing and causing personal injury.
Typically, these lift systems consist of a platform that can be folded and unfolded between a vertically oriented, stowed position in the vehicle and an unstowed, transporting position horizontally extending from the vehicle floor. From its unfolded or unstowed position, the platform can be raised and lowered between the vehicle's floor level and the ground level like an elevator. The lift of FIG. 1 is a typical wheel chair lift system. Most such prior art lift systems use essentially the general principles that are illustrated. The lift allows a person in a wheelchair to roll along the ground and onto the lift platform to be raised into the vehicle. The platform is then raised from ground level up to the vehicle's floor level. After reaching the floor level, the person rolls from the platform into the vehicle. Then the person operates the mechanism to cause the platform to pivot into the vehicle and stow the lift in the vehicle.
To minimize the cost and complexity of a wheelchair lift system, it is advantageous to perform the platform lifting function and the stowing function utilizing a single hydraulic cylinder or two or more cylinders 1 operated hydraulically in parallel, such as illustrated in FIG. 1.
FIG. 2 shows the fundamental mechanical structures of a typical wheel chair lift system that incorporates a hydraulic cylinder 1 to perform both the wheelchair lowering and lift functions and the platform deployment and stow functions. The system includes a first fixed vertical pillar 2 that is securely attached to the vehicle. A lifting platform 3 is attached to a second, vertically movable, vertical pillar 4 at a hinging pivot 5. A brace 9 is attached between the vertical pillar 4 and the platform 3 in such a fashion as to limit the range of motion of platform 3 around hinging point 5 so that it can pivot to no more than a 90° angle to the vertical pillar 4. The vertical pillars 2 and 4 are mechanically coupled to each other with two parallel equal length arms 6 and 7 that are hinged at their attachment points to the vertical pillars 2 and 4. The hydraulic cylinder 1, when operated, raises the platform 3 from ground level up to vehicle floor level. Whenever the platform 3 is raised above floor level, a stop 8 engages a platform protrusion 3a which directs the motion of the platform 3 around its hinging point 5 causing the platform 3 to fold, that is to pivot upwardly about its pivot axis 5 near its innermost edge until it reaches a substantially vertical orientation.
This operation is illustrated in more detail in FIG. 3 but the hydraulic cylinder is not illustrated in order to simplify the drawing and because it can be located in multiple optional positions. As known to those skilled in the art, the hydraulic cylinder or cylinders can be located to either push or pull in order to raise the lift, depending upon which obliquely opposite pivots for the arms 6 and 7 that it is connected to in the parallelogram arrangement that supports the platform. A hydraulic cylinder can also be attached separately from the pivots for the arms 6 and 7 since the purpose of a hydraulic cylinder is to raise and allow lowering of the vertical pillar 4 relative to the vertical pillar 2.
A wheelchair lifting cycle begins, as illustrated in FIG. 3A, with the wheelchair lift system fully deployed so that the platform 3 is resting at ground level. In this position a wheelchair can easily be rolled on to or off of the platform. Pumping fluid into the hydraulic lifting cylinder causes the second vertical pillar 4 and platform 3 to rise with respect to vertical pillar 2 from ground level towards the vehicle floor level as shown in FIG. 3B. The lifting cycle is completed when platform 3 reaches the vehicle's floor level as shown in FIG. 3C. In this position a wheelchair can easily be rolled between the pillars into or out of the vehicle.
Once the lift has served its purpose to raise the user to the vehicle floor level, the lift needs to be stowed. A stow cycle begins with platform 3 at vehicle floor level as illustrated in FIG. 3D. The mechanical structures are so arranged that after the platform reaches floor level, application of more force from the hydraulic cylinder causes the platform to pivot around its pivot point 5 because further vertical movement of the platform is limited by the floor level stop 8. Pumping fluid into the hydraulic cylinder causes the second vertical pillar 4 to rise with respect to vertical pillar 2 in turn forcing platform 3 to fold around pivot 5 as shown in FIG. 3E because the protruding part 3a of the platform 3 engages the stop 8, causing the platform to fold in against the pillars as the pillars 2 and 4 are driven together by the hydraulic cylinder, as shown in FIGS. 3D-3F. The stowing cycle is complete when platform 3 is fully recovered to its vertical stowed position as shown in FIG. 3F.
These operations are reversible. Releasing fluid from hydraulic cylinder when platform 3 is in the fully stowed position, as shown in FIG. 3F, allows the force of gravity to first cause the second vertical pillar 4 to descend with respect to the first vertical pillar 2 allowing platform 3 to unfold around pivot 5. The unstow operation is complete when platform 3 is fully deployed and is parallel to and level with the vehicle's floor as shown in FIG. 3C. From this position a wheelchair can easily be moved from the vehicle onto the platform. Releasing additional fluid from the hydraulic cylinder causes the second vertical pillar 4 and platform 3 to descend with respect to the first vertical pillar 2 from vehicle floor level to ground level. The platform lowering operation is complete when platform 3 reaches ground level as shown in FIG. 3A.
Turning now to the electrical and hydraulic circuitry, FIG. 4 illustrates a basic prior art hydraulic circuit and electrical controlling circuit for a wheelchair lift system described above although some conventional, prior art components and options are not included.
The hydraulic circuit includes a hydraulic lifting cylinder 11, an electric motor driven hydraulic pump 12, a normally closed, electrically energized, hydraulic fluid bypass valve 13 and a hydraulic fluid reservoir tank 14. A battery BAT is connected to a contactor 15 that operates as a power switch to control electrical current through the electric motor of the electric motor driven hydraulic pump 12. The electric motor is not directly switched on and off by a mechanical, hand-held switch because the motor currents are too large and would require an excessively large electrical cable in the user's hand to control the lift. So the separate contactor or power switch 15 is used. When electric power is applied to the hydraulic pump 12, fluid is pumped from the reservoir tank 14 to the lifting cylinder 11. Check valves internal to the hydraulic pump 12 prevent reverse hydraulic fluid flow through the pump. When power is applied to the bypass valve 13 and if the hydraulic lifting cylinder 11 is under pressure from a force applied to it, such as gravity, hydraulic fluid will return from the lifting cylinder 11 through the bypass valve 13 to the reservoir tank 14.
Low current switches 16, 17, 18, 19 and 20 control the power contactor 15. These include four separate hand control switches 17, 18, 19 and 20. Two of these switches, 17 and 18 can apply power to the contactor, closing its high current circuit and thereby applying current to the electrical motor to cause the motor to operate and develop hydraulic pressure for raising the lift. Two other switches 19 and 20 operate the bypass valve 13 causing fluid to drain from the hydraulic cylinder for its lowering movement. Each of the two sets of hand control switches is controlled by a fifth switch 16, and that fifth switch is mounted to the lift as a limit switch to be engaged and change state when the platform reaches the vehicle's floor level. Consequently, when the platform 3 is at ground level or at any intermediate position between the positions of FIGS. 3A and 3C, switch 16 is in the state illustrated in FIG. 5. When the platform is rising and arrives at the position of FIG. 3C, the switch 16 switches to the opposite state and is in that state at every position above that.
There are four distinct functions performed by the wheelchair lift system described above which are:
1. Raising the platform
2. Stowing the platform
3. Deploying the platform
4. Lowering the platform
When the platform 3 is at ground level, switch 16 can supply power to switches 18 and 19. Switch 18 controls raising the platform. If platform 3 is below floor level, switch 16 connects the battery positive terminal to switch 18. Manually closing switch 18 connects the battery positive terminal to power contactor 15 in turn switching battery positive to apply battery voltage to the hydraulic pump 12. Unless switch 18 is opened, the hydraulic pump continues to operate until the platform reaches floor level at which time switch 16 changes state and removes battery power from switch 18 and the power contactor 15. When it does, the circuit to the contactor 15 through switch 18 is opened which interrupts the motor current and automatically stops the ramp at that level. At that point the user gets off the lift platform and then wants to stow the lift.
The user initiates stowing of the lift by pushing the stow button, to close switch 17 which controls stowing the platform. Manually closing switch 17 connects the battery positive terminal to power contactor 15 in turn switching battery positive to the electric motor of the hydraulic pump 12. The hydraulic pump operates raising the platform 3 from the vehicle floor level position to the fully stowed position at which time the switch 17 is manually released by the user. Of course a limit switch can be included to assure that the electric motor ceases operation.
Switch 20 controls deploying the platform. If platform 3 is above floor level, switch 16 connects the battery positive terminal to switch 20. Manually closing switch 20 connects battery positive to the hydraulic bypass valve 13 operating it to cause hydraulic fluid to drain from hydraulic cylinder 11 to reservoir tank 14. The hydraulic cylinder 11 retracts until the platform reaches floor level at which time switch 16 changes state and removes battery power from switch 20 and the hydraulic bypass valve 13.
Switch 19 controls lowering the platform from the vehicle floor level. Switch 16 connects the battery positive terminal to switch 19. Manually closing switch 19 connects battery positive to the hydraulic bypass valve 13 operating the valve 13 causing hydraulic fluid to drain from hydraulic cylinder 11 to the reservoir tank 14. The hydraulic cylinder 11 retracts until platform 3 reaches ground level or switch 19 is released.
Safety is the first consideration in the operation of any wheelchair lift system. The failure of any single component, switch, sensor or control should not affect safe operation. Examining the electrical schematic of the typical wheelchair lift system depicted in FIG. 4 reveals several intrinsically unsafe design problems. First, the electrical contacts of power switch 15 could fail. Unlike low current control switches that can reliably operate for tens of thousands of cycles, high current power switches historically are much less reliable. Typically, in power switches the electrical contacts fail long before the mechanical actuating apparatus because of the relatively high currents they carry and the reactive loads to which they are connected. If the wheelchair platform were occupied during a lift cycle and the contacts in power switch 15 were to weld closed or shorted, the electric motor could not be de-energized and the platform would transition from the lift cycle directly to the stow cycle. In other words, the lift platform 3 with the occupant still on it would just keep moving past the vehicle floor level and begin to pivot to its stowed, vertical orientation.
Some wheelchair lift manufacturers have recognized the safety problem of the welded contacts and have added a second, series-connected power switch to circuits controlling hydraulic motors. Referring to FIG. 5, a first power switch 21 is shown in series with a second power switch 22 and having their respective control windings connected in parallel so both are opened and closed simultaneously. Except for the use of the dual power switches 21 and 22, the circuit of FIG. 5 is the same as the circuit of FIG. 4. The theory is that, if one contactor fails to open, it is probable that the other will open. That does reduce the probability of such a failure. As known to those in the electrical arts, the power switches can be interposed anywhere in the circuit extending from the power source to the electric motor and its return loop. Thus, they can be on the positive or negative side of a DC power circuit.
However, examining the electrical schematic shown in FIG. 5 reveals remaining safety problems. The dual contactor solution does not eliminate the possibility of an eventual failure because the second contactor is also likely to eventually fail. The predominant failure mode in the industry is stuck or welded contacts in these high power switches. If both power switches fail in a shorted mode, the result is the same cause of serious injury. In one scenario, a first switch would fail in a contact shorted mode at a time near the end of its normal life cycle. Lift operation would continue without any apparent problem until the second switch failed. Since both power switches are operated simultaneously, they both see the same number of mechanical cycles and they both see the same electrical loads and therefore both switches experience the same mechanical and electrical wear. After a first switch failure there is a higher probability of a second switch failure. Although using two series switches statistically increases the probable time to circuit failure, it does not prevent ultimate circuit failure and the resulting hazard.
It is therefore an object and feature of the invention to provide a failsafe improvement in the way these switches are controlled and operated to eliminate the possibility of having two switches fail at the same time, potentially causing the lift to fail and pass through to full stow mode causing injury.
Another object and feature of the invention is to provide a control method and circuit that is equally applicable to semiconductor power switches, and other mechanical switches for supplying power to an electrical load.
Yet another object and feature of the invention is to provide a method and circuit for controlling such power switches in other types of apparatus in which dual series connected switches supply electrical power to an electrical load.